1. Field of the Invention
The present invention relates to a color image forming apparatus and a color image forming method in the color image forming apparatus.
2. Related Background Art
Recently, a printer has become colorized and used for presentations by users in various ways. Particularly, a color page printer is attracting interest because of its low noise, high print quality and high print speed. A color laser printer which is one of such color printers is explained below.
The color laser beam printer is characterized by a first step of forming a latent image of a first color component by scanning a light beam on a photo-conductive medium in a main scan direction to conduct first development and then transferring it on a carrier such as a transfer drum, followed by second, third and fifth steps to transfer toner images of respective colors, and a step of retransferring the toner images collectively onto a recording sheet which is a recording medium at a predetermined timing to make a record of multi-color image.
FIG. 1 shows a sectional view of a conventional color laser beam printer. In FIG. 1, C denotes a mechanical unit of the color laser beam printer. Numeral 1 denotes a photo-conductor drum, numeral 4 denotes a charger, numeral 5 denotes a semiconductor laser, numeral 6 denotes a scanner motor, numeral 7 denotes a polygon mirror rotated by the scanner motor 6, numeral 8 denotes a lens and numeral 9 denotes a mirror. A light beam L which is on/off modulated by an image signal S 110 (hereinafter referred to as a VDO signal) is outputted from the semiconductor laser 5. The light beam L emitted from the semiconductor laser 5 is deflected by the polygon mirror 7, passes through the lens 8 and the mirror 9 and is directed to the photo-conductor drum 1.
Numeral 12M denotes a developing unit for developing a latent image by magenta color toner (M toner) in the first step. Numeral 12C denotes a developing unit for developing the latent image by cyan color toner (C toner) in the second step. Numeral 12Y denotes a developing unit for developing the latent image by yellow color toner (Y toner) in the third step. Numeral 12BK denotes a developing unit for developing the latent image by black color toner (BK toner). Numerals 20 and 21 denote sheet cassettes for containing record sheets P which are fed one by one by a sheet feed roller 14 or 15 of the sheet cassette selected by an image control unit to be described later.
Numeral 16 denotes a transfer drum which comprises a support 17 and a film 18. Numeral 10 denotes a cleaner for scraping off the toner image which is left undeveloped on the photo-conductor drum 1 at each completion of the transfer process. Numeral 13 denotes a fixing unit and numeral 19 denotes a sheet elect tray. Numeral 11 denotes a charger. Numeral 2 denotes a shield plate for providing a reference to a print timing on the transfer drum 16. Numeral 3 denotes a photo-interrupter. In order for the shield plate to block a light path of the photo-interrupter 3, a reference signal (hereinafter referred to as a TOP signal) is sent from the photo-interrupter 3 to an engine control unit 60 and an image processing unit 50 (see FIG. 2) to be described later for each revolution of the transfer drum 16, and a print start signal and a reference signal to a print operation are generated based on the TOP signal.
An operation of the printer of FIG. 1 is explained. First, the photo-conductor drum 1 is charged to a predetermined polarity and a predetermined voltage by the charger 4 and a photo-conductor drum 1 is scanned by the light beam L modulated by the VDO signal S110 to form a first electrostatic latent image. Then, the first electrostatic latent image is developed by the developing unit 12M to form a first toner image of the magenta color on the photo-conductor drum 1.
On the other hand, a predetermined bias voltage of the opposite polarity to that of the toner is applied to the transfer drum 16 and the first toner image is transferred to the film 18 on the transfer drum 16.
A second electrostatic latent image is then formed on the photo-sensitive drum 1 by the light beam L and it is developed by the developing unit 12C to form a second toner image of the cyan color on the photo-conductor drum 1. The second toner image is transferred to the transfer drum 16 at the position to which the first toner image was transferred.
Similarly, third and fourth electrostatic latent images are formed on the photo-conductor drum 1 and they are developed by the developing units 12Y and 12BK, respectively, to transfer the toner images of the yellow color and the black color to the film 18 on the transfer drum 16. The record sheet P is fed at a predetermined timing so that the toner images of four colors are collectively transferred to the film 18 on the transfer drum 16 and the toner images of four colors are formed on the record sheet P. In this manner, one page of VDO signal S110 is sequentially outputted to the semiconductor laser 5 for each process.
Then, the charger which separates the record sheet P having the four-color toner image transferred thereon from the transfer drum 16 discharges the stored charges on the record sheet P to reduce in-the-air discharge in the retransfer/separation.
FIG. 2 shows a configuration of a print system comprising the color printer described above and a single host computer. Numeral 100 denotes a host computer which generates print information and sends the print information to the printer through a host interface S1. Numeral 50 denotes an image processing unit of the printer which is arranged in a printer mechanism C to store dot data sent from the host computer 100 to a memory to be described later or monitors and controls a print command to a mechanism control unit 60 to be described later. Numeral 60 denotes the mechanism control unit which controls a control module, not shown, necessary for the print operation in the printer mechanism C and supplies the VDO signal to the semiconductor laser 5.
FIG. 3 shows a configuration of the image processing unit 50. Numeral 51 denotes a host interface control unit for controlling an interface with the host computer 100, numeral 52 denotes a CPU (central processing unit) which as a control center of the image processing unit, numeral 53 denotes a memory for a working area of the CPU 52 and holding the four-color dot data, numeral 54 denotes an engine interface unit for exchanging data with the mechanism control unit 60 and numeral 55 denotes a signal generator for generating a reference signal for a second page in two-sheet simultaneous printing.
A control operation of the image processing unit 50 is now explained. One page of dot data inputted from the host interface S1 is temporarily stored in the memory 53 by the control of the CPU 52. When one page of dot data is stored in the memory, the CPU 52 commands the start of the print operation to the mechanism control unit 60 through the engine interface unit 54. In response to the command, the mechanism control unit 60 starts the print operation and sends the TPO signal from the photo-interrupter 3 to the image processing unit 50 as a print start timing signal and further sends a horizontal synchronization signal, not shown, to the image processing unit 50 for each predetermined timing. Each time the image processing unit 50 receives the horizontal synchronization signal through the engine interface 54, it sequentially sends the dot data stored in the memory 53 to the mechanism control unit 60 through the engine interface unit 54.
FIG. 4 shows a timing chart of a send timing of the dot data sent from the image processing unit 50. Particularly, FIG. 4 illustrates the printing on the print sheet P having a sheet length along the feed direction of the record sheet P is substantially equal to the circumference of the transfer drum 16.
In FIG. 4, T1 and T2 denotes first and second print periods, respectively, Q denote a period between the print periods T1 and T2, B denotes a sheet feed operation period for one of the two cassettes shown in FIG. 1 and T4 denotes a period from the start of TOP signal in the fourth step to the start of sheet feed for transferring the toner images of the first and second pages on the transfer drum 16 to the record sheet. The printing of the first page data is made during the period T1, and the development of the second page dot data is made during the period T2.
In the prior art, since only one sheet of toner image is transferred during the transfer process by one revolution of the transfer drum 16, a long time is required when the same image is to be printed on a plurality of sheets.
Further, when different dot data is to be printed for each page, the printing is stopped each time to exchange and store the dot data. Accordingly, the waiting time Q is long as shown in FIG. 4. Particularly, as shown in FIG. 8, when a plurality of host computers 100a, 100b, 100c, 100e and 100f are connected to the host interface, the waiting time is longer than the waiting time for connecting the single host computer shown in FIG. 2. Recently, as the network is advanced, the connection configuration as shown in FIG. 8 is increasing.
In order to solve the increase of the waiting time, a second memory may be added to the image processing unit 50 so that the dot data is stored in the second memory while the dot data in the first memory 53 is sent to the mechanism control unit 60. However, such addition of the memory is very expensive and a time corresponding to approximately one revolution of the transfer drum is required before the mechanism control unit 60 is started up once it is stopped. Thus, the waiting time increases.